Silver naturally tarnishes in the atmosphere, especially in the presence of sulphur. Industrial atmospheres and natural digestive processes are important sources for the tarnishing of silver. When silver is tarnished, sulphides, oxides or carbonates are formed on the surface of silver. The tarnishing of silver and silverware is a problem for example for the utility article, jewelry and giftware industries as well as to the end users of the silver products. Tarnishing degrades the appearance of the product as a layer or spots of black or dark grey colour are formed. It can be removed but this is usually a laborious process, and the process may affect the appearance of the product negatively. Also in technical applications the tarnishing of the silver reduces the optical properties, such as reflectivity, of silver and silver products and parts.
Methods for preventing tarnishing in advance are known in the prior art. One existing method for preventing tarnishing comprises using silver alloys that are designed with resistance to oxidation and involve the mixing of special additives, such as silicon or germanium, with silver. Another existing method for preventing tarnishing comprises coating pure silver using rhodium. One of the problems associated with the above arrangement using silver alloys is that the method requires all factors to be carefully controlled during manufacture, like using extremely pure new metal and accurate temperature control in melting and annealing. As a result, the manufacturing process and the equipment to carry out the process are very expensive to set up. The cost is also prohibitive in the method for coating silver with rhodium. Further, the rhodium coating has a blue-white shade, and thus a silver product coated with rhodium may become visually different from pure silver.
Other existing methods for preventing tarnishing of silver comprise coating the finished silver product, article or part with a method that provides a layer of material on the silver product, article or part which prevents or restrains tarnishing on the finished silver product, article or part. These kind of prior art methods comprise varnishing the silver products. The problem with these known coating methods is that the coating layer is not uniform over the whole product or part of the product that has been coated. The thickness variations in the coating layers over the silver product cause colour variations, for example due to interference, or other optical alterations, which are not preferable. These known methods also produce relatively thick layers of coating materials on the silver products. This further has a negative influence on the appearance of a silver product. Varnish may also yellow and peel off. Thus, the known methods for preventing tarnishing of a silver product do not provide a uniform and substantially to human eye invisible coating, but a non-uniform coating and/or a coating producing discoloration of a silver product.